Memory devices such as an SD™ card are classified into a plurality of classes to facilitate matching between the performance of a memory device and that required by a host device. A speed class provides a method of classifying the performance of memory devices by speed class numbers and calculating the performance of the memory devices.
The speed class controls the write procedure using specific commands. To do memory write while maintaining performance, sequential write is performed in an allocation unit (to be referred to as an AU or sequential write area hereinafter) that is the physical memory area of a memory device. Data needs to be written from the start of an allocation unit. For this reason, an allocation unit in which data has been written partway cannot be used for data recording. Therefore, it is desired to provide memory device and host device capable of using an allocation unit in which data has been written partway and improving the utilization efficiency of allocation units.